This invention relates to the technical field of collimating plates for collimating diffused light and backlight systems utilizing them. More particularly, the invention relates to a collimating plate that can produce adequately convergent collimated light and which hence is particularly suitable for use as a backlight for liquid-crystal displays, and a backlight system utilizing it.
The use of liquid-crystal displays (LCDs) as a display for word processors and computers is rapidly increasing today. The use of LCDs as a monitor in ultrasonic, CT and MRI diagnostic apparatus is under review. Conventionally, these medical diagnostic apparatus have primarily used CRTs (cathode-ray tubes) as a monitor.
LCDs have many advantages such as ease in size reduction, small thickness and lightweightness. On the other hand, they have poor viewing angle characteristics (narrow viewing angle) in that as the viewing direction or angle changes, the contrast ratio of an image decreases sharply and the gradation also reverses to have the image look differently. As a result, depending on the position of the viewer, the image cannot be viewed correctly.
In medical applications as described above, diagnosis with monitors is based on the difference in image density, so not only is it required to produce an image of high contrast ratio but at the same time, incorrect recognition of an image can cause wrong diagnosis or inconsistency in the results of diagnoses. Under these circumstances, it is required that high-contrast image be displayed over a wide range of viewing angles. Another problem peculiar to medical monitors is that image is usually displayed in monochrome (black and white colors) and, hence, suffers considerable drop in contrast as the viewing angle varies.
LCDs adapted for viewing over a wide range of angles include those operating in an IPS (in-plane switching) mode and a MVA (multi-domain vertical alignment) mode. However, even these LCDs do not have a sufficiently wide range of viewing angles to satisfy the requirement of monochromatic images, especially ones that are used in medical applications.
A LCD is known that can produce image of good contrast ratio over a wide range of viewing angles. It is backlighted with collimated light (which travels straight) and the light passing through the liquid-crystal panel is diffused with a diffusing plate.
In order to realize a wide-angle LCD, the diffused light issuing from the backlight source has to be condensed sufficiently to produce adequately collimated light. In particular, in order to realize a LCD that has a sufficiently wide range of viewing angles to be useful in medical applications, it is preferred to use collimated light the intensity of which drops by 50% at angles of no more than .+-.15 degrees.
Collimated light is usually produced by making diffused light convergent with a condensing sheet. The collimated light produced from conventional condensing sheets is reduced in intensity by 50% at angles of about .+-.20 degrees and it has been difficult to produce collimated light that practically serves to backlight LCDs for medical use.